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These images taken by NASA's Hubble Space Telescope show four members of the Virgo cluster of galaxies, the nearest large galaxy cluster to Earth.

They are part of a survey of globular star clusters in 100 of Virgo's galaxies. Globular clusters, dense bunches of hundreds of thousands of stars, have some of the oldest surviving stars in the universe. Most of the star clusters in the Virgo survey are older than 5 billion years.

The galaxy at top, left, NGC 4660, contains 205 globular clusters. The galaxy at top, right, NGC 4458, has 72. Both galaxies possess a typical number of globular clusters for their size and brightness.

IC 3506, at bottom, left, is a dwarf galaxy near Virgo's central hub that contains 31 globular clusters. By contrast, the dwarf galaxy VCC 1993, at bottom, right, has no clusters and resides in Virgo's outskirts.

The Hubble study found evidence that globular clusters are more likely to form in dense areas, where star birth occurs at a rapid rate, instead of uniformly from galaxy to galaxy. Dwarf galaxy IC 3506, for example, resides near Virgo's dense center and has four times as many clusters for its size and brightness as NGC 4660 and NGC 4458.

Hubble's "eye" is so sharp that it was able to pick out the fuzzy globular clusters, which, at that distance, look like individual stars bunched up around the galaxies, instead of groupings of stars.

Comprised of over 2,000 galaxies, the Virgo cluster is located about 54 million light-years away.

Astronomers made these composite images from the advanced camera's full field-of-view observations. They also used modeling data to fill in a narrow gap between the camera's detectors. The images were taken from December 2002 to December 2003.